Method for actuating a reversible belt pretensioner

ABSTRACT

Method for actuating a reversible belt pretensioner for pretensioning a belt webbing, the belt pretensioner having a drive which acts on the belt reel, and the seat belt having, as a belt extraction lock, a locking device which acts on the belt reel in the unwinding direction and whose locking can be released only if the belt reel is rotated through a specific angle of rotation in the winding-on direction. After the triggering of the belt pretensioner, the drive of the belt pretensioner for opening the locking device is actuated in an opening mode such that firstly, in a winding-on phase, the belt reel rotates through the specific angle of rotation in the winding-on direction. In a subsequent holding phase, the drive of the belt pretensioner is actuated for a predefinable holding time T H  such that the belt reel is held in its angular position.

BACKGROUND AND SUMMARY OF THE INVENTION

This application claims the priority of International Application No.PCT/EP01/14328, filed Dec. 6, 2001, and German Patent Document No. 10061 040.4, filed Dec. 8, 2000, the disclosures of which are bothincorporated by reference herein.

The present invention relates to a method for actuating a reversiblebelt pretensioner.

Seat belts which are nowadays usually installed in motor vehicles have amechanism for automatically winding on the loose belt webbing onto abelt winding-on roller which is arranged on a belt reel. The automaticwinding on ensures that the belt which is being worn rests looselyagainst the body of the vehicle occupant, and when the seat belt is notin use it is rolled up on the belt winding-on roller. The unwinding ofthe belt from the belt winding-on roller takes place counter to a torquewhich is generated by a return spring and is also possible when the beltis being worn in order to permit the vehicle occupant to make a largelyfree movement.

Furthermore, a locking device which acts as a belt extraction lock isusually present. This locking device is triggered by an actuation devicewith a belt-webbing sensitive and vehicle-sensitive sensor when the beltis being unwound rapidly from the belt winding-on roller, starting froma predefined acceleration of the belt winding-on roller in the unwindingdirection or starting from a predefined acceleration of the vehicle. Thebelt winding-on roller is fixed in the present position in such a waythat the belt is prevented from unwinding. A customary locking device iscomposed, for example, of a locking toothing of the belt reel and alocking latch which is pivotably mounted and can be pivoted into thelocking toothing of the belt reel by means of the actuation device. Thetoothings on the belt reel and the locking latch are of self-lockingdesign, causing the extraction of the belt to be locked for as long astensile loading is applied to the belt. If the belt is relieved of thistensile loading, the locking mechanism can open, that is to say it canbe returned into its inactive state. This return takes placemagnetically, electromotively, or usually mechanically, for example bymeans of a restoring spring on the pivotably mounted locking latch. Sucha locking device and a method for actuating a belt pretensioner withsuch a locking device are described in the post-published German PatentDocument No. DE 100 61 040 A1.

In addition, in many seat belts there is a belt pretensioner whichincreases the protective effect of the seat belt. The belt pretensionerhas a drive which can be actuated in order to drive a mechanical devicewhich shortens the loose belt webbing and/or pulls the vehicle occupantbackwards. In the case of a pyrotechnic energy store, for example achemical substance is caused to undergo an exothermic reaction by meansof a firing element. In this reaction, a gas flow, which drives themechanical device, is generated. The driven device is, or can be,mechanically connected to the belt reel, for example by means of aclutch. By means of this connection, the mechanical device exerts atorque on the belt reel. Owing to this torque, the belt reel rotateswith the belt winding-on roller arranged on it and pretensions theunwound belt webbing.

Apart from the usually pyrotechnic belt pretensioners which are usednowadays in motor vehicles, it is also possible to use reversible beltpretensioners in vehicles which can be triggered repeatedly, evenrapidly in succession. These reversible belt pretensioners can havedifferent drives, for example such a belt pretensioner can be driven byan electric motor which acts on the belt reel permanently or in a waywhich can be controlled by means of a clutch. Other reversible beltpretensioners are driven with compressed air from a pressure accumulatoror by means of a tensioned spring, it being possible to refill thepressure accumulator during the travel mode and to stress the springagain during the travel mode.

Reversible belt pretensioners which are driven in this way permit theseat belt to be pretensioned to a predefinable degree, with apredefinable velocity and for a predefinable period of time. Themultiple triggering capability of the reversible belt pretensionerpermits it to be triggered preventatively. Preventative triggering meansthat the belt pretensioner is triggered in safety-critical travelsituations which are detected, for example, by vehicle-movement dynamicssensors or vehicle-surroundings sensors or which are determined by meansof the evaluation of the actuation of the brake pedal, of the steeringangle or of a driver observation. In addition to preventativetriggering, a reversible belt pretensioner can also be used for hapticwarning of the driver in safety-critical situations. When there ispreventative triggering of the belt pretensioner which may occur beforea collision is detected or when the belt pretensioner is triggered forwarning purposes, it is desirable that the belt should rest looselyagainst the vehicle occupant again after pretensioning has taken place,after the hazardous situation has ended and when the normal travel modehas been safeguarded. The normal travel mode is safeguarded if theevaluation of the situation by a control unit or a hazard computer doesnot reveal that there is a safety-critical situation or if the conditionwhich is necessary for the triggering of the belt pretensioner is nolonger fulfilled. Such a condition may be the presence or the absence ofa specific signal on a data bus or a data line.

After reversible pretensioning of the seat belt by means of a reversiblebelt pretensioner it is possible that the belt extraction lock hasbecome active owing to a belt-webbing-sensitive sensor or avehicle-sensitive sensor which actuate the locking device electricallyor mechanically. Examples of belt-webbing-sensitive sensors are amechanical centrifugal-force sensor in the belt winding-on mechanism, anelectromechanical centrifugal-force sensor and an electronic beltextraction sensor which senses the extraction velocity of the beltwebbing or its acceleration. In particular, a belt-webbing-sensitivesensor can respond if, after belt pretensioning, the pretensioned beltis released which is under tensile loading. This tensile loading of thepretensioned belt is in particular dependent on the degree of thepretensioning operation which has previously occurred and on the sittingposition of the vehicle occupant before the pretensioning operation. Asa result of this tensile loading, the belt is unwound again from thebelt winding-on roller after the pretensioning operation and once thetorque exerted on the belt reel by the belt pretensioner drivedecreases. If the unwinding of the belt webbing takes place too quickly,the belt-webbing-sensitive sensor responds and the locking device isactuated—usually mechanically—so that it closes. That is to say the beltextraction lock becomes active. The belt extraction lock can alreadybecome active if the belt-webbing-sensitive sensor responds owing to thewinding-on operation during the pretensioning of the belt, or thevehicle-sensitive sensor responds owing to the movement of the vehicle.If the belt extraction lock is active, it is no longer possible tounwind the pretensioned belt webbing and the freedom of movement of thevehicle occupants is greatly restricted. The belt webbing is thereforeto be released again if the cause of the triggering of the beltpretensioner no longer applies or if a normal travel mode is ensured. Inorder to release the belt webbing again, in locking devices which arecustomary today in motor vehicles, the locking latch has to be releasedfrom the locking toothing. The belt webbing can be released only if thevehicle-sensitive and the belt-webbing-sensitive sensor do not actuatethe locking device in order to close it.

The object of the invention is to improve the convenience when applyinga reversible belt pretensioner.

In the method according to the invention for actuating a beltpretensioner, the belt pretensioner is actuated to open the lockingdevice in an opening mode after the belt pretensioning operation whichhas taken place owing to a hazardous situation in order to protect avehicle occupant, and when a predefinable condition, which is indicativeof the ending of the hazardous situation and thus indicative of a normaloperating state of the vehicle, applies. This actuation takes place insuch a way that the belt reel firstly rotates, in a winding-on phase,through a specific angle of rotation in the winding-on direction, as thelocking device can open only if the belt webbing no longer exerts anyforce on the locking mechanism, and for example in the case of a lockingdevice with undercut locking toothing, a rotation of the belt reel atleast through the undercut of the teeth takes place, the belt webbingbeing slightly wound on. In a holding phase which follows the winding-onphase, the drive of the belt pretensioner is actuated for a predefinableholding time T_(H) in such a way that the belt reel is held in itsangular position during this holding time T_(H), that is to say the beltreel does not rotate and the belt is not unwound or wound on. Here, thelocking device is relieved, for a predefinable holding time T_(H) of thetensile loading which originates from the pretensioned belt which isresting tightly against the vehicle occupant. After the holding phase,the belt webbing is released in an unwinding phase in order to unwind.The advantage of the invention is that the holding time T_(H) can bedefined in such a way that up to its end the belt webbing can be unwoundwith a desired probability.

As the locking device has to be opened, and the vehicle-sensitive sensormust not respond so as to be able to unwind the belt webbing, theholding time T_(H) is, for example, predefined in such a way that theseconditions are fulfilled with the predefined probability.

In one refinement of the method, the holding time T_(H) is predefined insuch a way that it is ensured that at the latest up to the end of theholding time T_(H), the vehicle-sensitive sensor of the belt extractionlock permits the opening of the locking device. This ensures that theactuation of the belt pretensioner drive is terminated, and the motorcurrent of the belt pretensioner drive motor reduced, only if the beltreel rotates in the unwinding direction and can release the belt webbingfor unwinding, without the belt extraction lock being active. A repeatedactuation of the belt pretensioner drive which is necessary to releasethe locking device as a result of an active belt extraction lock, and anassociated repeated belt webbing winding-on operation, which coulddisturb or irritate the vehicle occupant, are prevented.

In one development of the method, a variable which is indicative of theopening of the locking device is registered. The holding time T_(H) willlast at least until it is determined that the locking device is opening,by means of the variable which is indicative of the opening of thelocking device. This refinement has the advantage that the holding timeT_(H) does not need to be predefined, independently of the situation,with such a magnitude that in all, or at least the most frequentlyoccurring, situations, the opening of the locking device is ensuredwithin the holding time T_(H). The holding time can as a result beshortened significantly depending on the travel situation, in order toincrease the convenience for the vehicle occupants.

A variable which is indicative of the opening of the locking device is,for example, the signal of a contact switch on the locking device whichcloses a contact precisely if the locking device assumes one of the twostates Opened or Closed, and which opens the contact if the lockingdevice assumes the other of the two states.

In one refinement of the method, the holding time T_(H) is predefined asa function of at least one vehicle-movement-dynamics parameter. Thepredefined parameter or parameters are registered, and the holding timeT_(H) is determined in accordance with the predefined function by meansof the registered parameter or parameters. In a simple case, thefunction can, for example, be such that in the case of full braking froma velocity above a velocity threshold value, the holding time T_(H) ishigher than in the case of full braking from a velocity below thevelocity threshold value. The advantage of this refinement is that theholding time T_(H) is adapted to the respective situation.

In one refinement of the method, the longitudinal acceleration or itsprofile is used as a parameter for determining the holding time T_(H).The longitudinal acceleration of the vehicle which occurs, for example,during a braking maneuver, is registered and the belt pretensioner isactuated in such a way that the belt reel is held, by the beltpretensioner drive, in the position in which the locking device canopen, until the registered longitudinal acceleration is smaller for apredefinable time period than a predefinable threshold value. Theadvantage of this refinement of the method is that, with the vehiclelongitudinal acceleration, a variable is used which also influences thelocking device of a belt system: if the longitudinal acceleration isabove a predefined threshold, the locking mechanism cannot open owing toan acceleration-sensitive sensor which responds, for example, startingfrom a vehicle acceleration of 0.3 g. Instead of the longitudinalacceleration (or its profile) or preferably in addition to thelongitudinal acceleration, the transverse acceleration (or its profile)can also be used in the same way.

In another refinement, in order to determine the time period duringwhich the belt reel is held in a position in which the locking devicecan open, a brake pressure (or its profile) and/or a coefficient offriction between the tire and road and/or the vehicle's own velocity (orits profile) and/or the position of the accelerator pedal and/or thesteering angle and/or the yaw angle and/or the transverse accelerationare registered. Alternatively, or in addition hereto, a signal of abrake booster system or a signal of a vehicle-movement dynamics controlsystem is registered and used to determine the holding time T_(H).

In one simple refinement, the holding time T_(H) is a function of thelongitudinal acceleration, in which case, when there is a change inacceleration from negative acceleration (deceleration, for example dueto a braking intervention) to positive acceleration (for example due toactivation of the accelerator pedal), the holding time T_(H)=0 is set.

In a further refinement of the method, the holding time T_(H) is afunction of the velocity of the vehicle and of a signal of a brakebooster system. The outputting of this brake booster signal by the brakebooster system brings about full braking and triggering of thereversible belt pretensioner in order to protect a vehicle occupant. Theholding time T_(H) is then equal to a first time period T₁ if, after thedisappearance of this brake booster signal and/or after termination ofthe full braking, the vehicle has a velocity which is less than or equalto a velocity threshold value, and the holding time T_(H) is equal to asecond time period T₂ if, after the disappearance of the brake boostersignal, the vehicle has a velocity which is higher than the velocitythreshold value. The velocity threshold value is preferably equal tozero. A plurality of different velocity threshold values and a pluralityof holding times T₁ to T_(n) can also be predefined in order to refinethe method.

In one simple example, a distinction is made between two cases by meansof the velocity threshold value zero: if full braking takes place up tothe stationary state of the vehicle, the full braking will be consideredto be terminated starting from the time from which the vehicle no longerhas any intrinsic velocity. The intrinsic velocity when the vehicle isin a stationary state is equal to zero, that is to say equal to thevelocity threshold value so that the holding time is equal to T₁. If thefull braking does not take place up to the stationary state of thevehicle, the vehicle has an intrinsic velocity greater than zero, andthe holding time is equal to T₂, after the disappearance of the brakebooster signal. As the vehicle takes up energy in the chassis duringfull braking with sufficient friction on the underlying surface, thevehicle oscillates in the stationary state for a short time (up toapproximately1 sec). During this time, accelerations occur on thechassis and they allow the belt extraction lock to become active owingto the vehicle-sensitive sensor, so that the unwinding operation cannottake place. The holding time T₁ is selected to have a correspondingmagnitude, for example equal to T₁=1 sec. If the full braking does nottake place as far as the stationary state of the vehicle, theseoscillations do not take place or only, take place very weakly, which isdue to continuous loosening of the brake linings which takes place fordesign reasons in contemporary vehicles. The holding time T₂ canconsequently be selected to be significantly shorter, and is for exampleT₂=0.3 sec.

In the holding mode, the belt pretensioner drive is actuated in such away that the belt reel is held, by the belt pretensioner drive, in aposition in which the position of the toothing against a locking latchand a gear wheel which is located on the belt reel permits the openingof the locking device.

The belt reel is preferably rotated by means of the belt pretensionerdrive only just until a position in which the locking device can open isreached for the first time. This position can be determined byregistering the angle of rotation of the belt reel, for example by meansof an angle-of-rotation sensor, or the profile of the motor current isevaluated. The rise in force in the belt which is brought about by theactuation in order to open the locking device is kept as small aspossible by rotating the belt reel as little as possible.

In addition, it is advantageous if the actuation of the beltpretensioner takes place in such a way that, in an unwinding phase whichfollows the holding phase, the torque of the belt pretensioner drive isnot reduced suddenly but rather slowly. As a result, the unwindingoperation takes place in such a way that the belt-webbing-sensitivesensor does not respond and the locking device does not become activeagain. The slow reduction of the torque takes place, for example, bycontinuously reducing the motor current of an electromotive beltpretensioner. After the unwinding phase, the vehicle occupant is in thestate in which he is wearing a belt with the belt resting looselyagainst him.

BRIEF DESCRIPTION OF THE DRAWINGS

One advantageous embodiment of the method according to the inventionwill be described below in more detail with reference to the drawing, inwhich:

FIG. 1 shows the profiles of the belt pretensioner drive force F(unbroken line/dashed line) and the belt travel s (dotted) over time tduring the execution of the method according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In FIG. 1, the profiles are represented only qualitatively and therelationships between the individual time intervals between tworespective times t_(n) and t_(n+1) do not correspond to those during anactual method sequence.

In the figure, the times to designate the start, and t₇ the end, of themethod for actuating a reversible belt pretensioner.

The reversible belt pretensioner which is used to describe the figureis, for example, an electromotively driven belt pretensioner. However,alternatively, a pneumatically or hydraulically driven belt pretensionercan also be used. In the case of an electromotive belt pretensioner, themost important directly controllable variable is the motor current whichflows through the drive motor. For this reason, reference is made belowto the motor current I. This is closely associated with the beltpretensioner drive force F which the belt pretensioner drive exerts onthe belt. An increase in the motor current generally also brings aboutan increase in the belt pretensioner drive force F.

The belt pretensioning for protecting a vehicle occupant starts at thetime t₀. At this time t₀, the vehicle is in a normal operating state andthe belt pretensioner is consequently in the initial state, that is tosay it is inactive. Correspondingly, the motor current which flowsthrough the belt pretensioner drive motor in the initial state is I=0.At the time t₀, a control signal of a brake booster system which causesthe reversible belt pretensioner to be triggered in order to protect avehicle occupant, is then registered. The belt pretensioner drive motorpretensions the belt with a predefinable high pretensioning force, thatis to say the motor current is I=I_(S), it being possible to adapt themotor current or the belt pretensioner drive force to a hazardoussituation and/or to determined vehicle occupant parameters as a functionof the embodiment of the belt pretensioner. The belt force rises to apretensioning value, and the belt length which is unwound from the beltreel drops to a minimum value.

The end of the actual belt pretensioning, which serves to protect thevehicle occupant, is defined by a predefinable time interval t₂−t₀, andis additionally or alternatively associated with a predefinablecondition, for example a threshold value for the acceleration of thevehicle or the force of the belt. In this pretensioning interval, themotor current rises up to the time t₁ in order to remain at thepredefinable pretensioning value I_(S) for a brief time period t₂−t₁.The time period t₂−t₁ can also be 0 sec, so that the rising phase of themotor current is followed immediately by the phase between the times t₂and t₃ in which the motor current is reduced to a restraining levelI=I_(R). This restraining level of the motor current can be relativelyhigh (dashed profile between t₃ and t₄), the extraction of the beltbeing largely prevented by the holding force of the drive motor of thebelt pretensioner. However, the restraining level of the motor currentis preferably relatively low (unbroken profile between t₃ and t₄). Partof the restraining force is then generated by the frictional forceswhich occur, for example, at the belt deflection roller. The motorcurrent can additionally be reduced, for example, as far as I=I_(R=)0 inthe event of a vehicle acceleration above 5 m/s². Undesired extractionof the belt between the times t₃ and t₄ is almost completely preventedin this case (I_(R)<<I_(S)) by the locking device, with the exception ofslight unwinding of the belt webbing until the belt reel reaches aposition in which the locking device engages and prevents furtherextraction of the belt.

By means of parameters such as the velocity of the vehicle, theactivation of the brake pedal, the activation of the accelerator pedalor the activation of the steering wheel, the end of a hazardoussituation and a normal operating state of the vehicle are determined,for example when the brake pedal is released after full braking or thevehicle comes to a standstill.

After a normal operating state of the vehicle is determined at the timet₄, and if appropriate further conditions are fulfilled, the drive ofthe belt pretensioner is actuated in the opening mode. The motor currentof the drive motor of the belt pretensioner is regulated to a holdingcurrent strength I=I_(H) for a predefinable holding time T_(H)=t₆−t₅.The motor current is regulated in the opening mode in such a way thatbetween the times t₄ and t₅ the belt reel rotates in the unwindingdirection until the position of the belt reel permits the locking deviceto open, the belt reel being unwound by a small amount. For thispurpose, in a customary locking device which is composed of a lockinglatch and a complementary toothing on the belt reel, the undercut teethof the locking latch and toothing must no longer overlap. The minimumangle of rotation is given by the angle of the undercut in such alocking device. At the time t₅, the belt reel has rotated through thenecessary angle of rotation which can be registered by means of anangle-of-rotation sensor or determined by evaluating the motor current.

In the particular case in which even after the hazardous situation hasended, a vehicle occupant is held in the opening mode with a high forceby the belt webbing, the holding current I_(H) may be higher than theoriginal pretensioning current I_(S).

So that the locking device can open, or so that it does not close againwhen the belt webbing unwinds, further conditions must be fulfilled. Forexample, the acceleration of the vehicle must be below a predefinedacceleration threshold value so that the acceleration-sensitive sensordoes not respond. After full braking this is frequently the case onlyafter a brief transient recovery phase of the vehicle. For this reason,the holding time T_(H)=t₆−t₅ is determined as a function of the velocityof the vehicle and a brake booster signal and T_(H)=T₁ is set if, owingto the disappearance of the brake booster signal at the time t₄, thevelocity of the vehicle is equal to zero when the opening mode isinitiated. If the travel velocity is greater than zero when the openingmode is initiated, the holding time T_(H)=T₂ is set, where T₁>T₂. At thetime t₆, it is ensured that all the conditions for opening the lockingdevice are fulfilled, and the belt webbing is released in the unwindingphase between the times t₆ and t₇ for unwinding. At the time t₇ thereversible belt pretensioner is in the initial state again.

Two or more drive devices may be provided as the belt pretensionerdrive. In particular, a second electric motor may be provided inaddition to a first electric motor, the first electric motor or thefirst drive device being configured specially for the belt pretensioningin a hazardous situation, and the second electric motor or the seconddrive device being configured specially for the opening mode.

1. A method for actuating a reversible belt pretensioner forpretensioning a belt webbing wound around a belt reel of a seat belt ina motor vehicle, the belt pretensioner having a drive which acts on thebelt reel, and the seat belt having, as a belt extraction lock, alocking device which acts on the belt reel in an unwinding direction andis released only if the belt reel is rotated through a specific angle ofrotation in a winding-on direction, comprising the steps of: triggeringthe belt pretensioner by a hazardous situation; satisfying a predefinedvehicle operating state which indicates an end of the hazardoussituation; and actuating the drive of the belt pretensioner for openingthe locking device in an opening mode wherein in a first winding-onphase the belt reel is rotated through the specific angle of rotation inthe winding-on direction to an angular position and wherein in asubsequent, holding phase the drive of the belt pretensioner is actuatedfor a predefinable holding time (T_(H)) in such a way that the belt reelis held in the angular position.
 2. The method according to claim 1,wherein the holding time (T_(H)) is predefined in such a way that avehicle-sensitive sensor of the belt extraction lock permits the openingof the locking device up to an end of the holding time (T_(H)).
 3. Themethod according to claim 1, wherein the holding time (T_(H)) ispredefined as a function of at least one vehicle-movement dynamicsparameter, wherein the parameter necessary for determining the holdingtime (T_(H)) is registered, and wherein the holding time (T_(H)) isdetermined by the registered parameter.
 4. The method according to claim1, wherein the holding time (T_(H)) is a function of a vehiclelongitudinal acceleration.
 5. The method according to claim 1, whereinthe holding time (T_(H)) is a function of a vehicle transverseacceleration.
 6. The method according to claim 1, wherein the holdingtime (T_(H)) is a function of a coefficient of friction between a tireof the vehicle and a road surface.
 7. The method according to claim 1,wherein the holding time (T_(H)) is a function of a brake pressure. 8.The method according to claim 1, wherein the holding time (T_(H)) is afunction of a velocity of the vehicle.
 9. The method according to claim1, wherein the holding time (T_(H)) is a function of a yaw rate of thevehicle.
 10. The method according to claim 1, wherein the holding time(T_(H)) is a function of a steering angle of the vehicle.
 11. The methodaccording to claim 1, wherein the holding time (T_(H)) is a function ofa signal of a vehicle-movement dynamics control system.
 12. The methodaccording to claim 1, wherein the holding time (T_(H)) is a function ofa signal of a brake booster system.
 13. The method according to claim 1,wherein the holding time (T_(H)) is a function of a velocity of thevehicle and of a signal of a brake booster system, the brake boostersignal bringing about full braking and triggering of the reversible beltpretensioner in order to protect a vehicle occupant, and the holdingtime (T_(H)) being equal to a first time period (T₁) if, after adisappearance of the brake booster signal and/or after a termination ofthe full braking, the vehicle has a velocity which is less than or equalto a velocity threshold value, and the holding time (T_(H)) is equal toa second time period (T₂) if, after the disappearance of the brakebooster signal, the vehicle has a velocity which is higher than thevelocity threshold value.
 14. The method according to claim 13, whereinthe first time period (T₁) is longer than the second time period (T₂).15. The method according to claim 13, wherein the velocity thresholdvalue is equal to zero.